the answer is C. 51 because we can see that all of the numbers are divisble by 5 except for 51
an easy way to tell is if the number ends in 0 or 5, it is divisible by 5
51 is the only one that doesn't end in 0 or 5 so it isn't divisible by 5 (it's divisible by 3 and 17)
the answer is C. 51
